Identification of cryptotephra horizons in a North East Atlantic marine record spanning marine isotope stages 4 and 5a (∼60,000-82,000 a b2k)

Tephrochronology is one of the few techniques that offer considerable, but as yet largely unrealised, potential for the correlation of North Atlantic palaeoclimatic archives spanning the MIS 4 and 5a substage period - a phase of pronounced global cooling during the initiation of the last glacial period. In this investigation, two previously unidentified rhyolitic tephra horizons believed to be sourced from Iceland were isolated within the giant piston core MD04-2822, retrieved from the Rockall Trough, North East Atlantic. One horizon was deposited at the beginning of Greenland Stadial 19 (similar to 70,150 a b2k) and the other around the cooling transition at the end of Greenland Interstadial 20 (similar to 75,320 a b2k). Density separation, EPMA and LA-ICP-MS techniques were utilised for the identification of these cryptotephra within the fine-fraction sediment (<80 pm diameter) to aid the discovery of primary deposits and to assess the role of mixing processes. An exploration of the geochemical homogeneity, co-variance of shard concentration with IRD and shard size distribution of the two tephra horizons rules out ice-rafting as a potential transport process and instead suggests deposition from primary airfall or via sea-ice rafting of airfall material. The latter processes would not cause a significant delay in deposition following a volcanic eruption due to more rapid transportation to depositional sites. Therefore, these horizons are potentially valuable chronostratigraphic markers for the GI-19 and GI-20 climatic events, two significant events that occurred around the transition between MIS 5a and 4. (C) 2011 Elsevier Ltd and INQUA. All rights reserved.